/**
 * Copyright (C) 2010-2013 Alibaba Group Holding Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.rocketmq.client.impl.consumer;

import java.util.concurrent.ConcurrentHashMap;

import com.alibaba.rocketmq.common.message.MessageQueue;

/**
 * Message lock,strictly ensure the single queue only one thread at a time consuming
 * 
 * @author shijia.wxr<vintage.wang@gmail.com>
 * @since 2013-6-25
 */
public class MessageQueueLock {
	private ConcurrentHashMap<MessageQueue, Object> mqLockTable = new ConcurrentHashMap<MessageQueue, Object>();

	public Object fetchLockObject(final MessageQueue mq) {
		// 2.1）根据MessageQueue对象从MessageQueueLock.mqLockTable: ConcurrentHashMap<MessageQueue, Object>中获取Object对象；
		Object objLock = this.mqLockTable.get(mq);
		if (null == objLock) {
			// 2.2）若获取的Object对象为null，
			// 则创建一个Object对象
			objLock = new Object();
			// 为了防止并发采用putIfAbsent方法将objLock存入mqLockTable列表中
			Object prevLock = this.mqLockTable.putIfAbsent(mq, objLock);
			if (prevLock != null) {
				// 若已经有该MessageQueue对象，则返回已经存在的Object对象
				objLock = prevLock;
			}
		}

		// 若获取到Object对象不为null，则直接返回该对象
		return objLock;
	}
}
